This invention relates to a coreless motor.
Recently, a coreless motor of the type in which a coreless rotor having a pancake coil on an insulating substrate in a one-layer or multi-layer fashion, has been developed. The rotor of this type is formed, for example, by printing a coil pattern, or attaching a coil pattern of electroconductive foil, onto a light-weight insulating coil such as synthetic resin etc. Since such coreless rotor is light in weight and small in inertial force, if it is used, for example, in a tape driving motor of a tape recorder calling for such features, a rise in the rotation of a capstan apindle during the starting of the motor as well as a rapid stopping of a tape in a desired position can be smoothly effected. However, if the center of gravity of the rotor is deviated due to, for example, the eccentricity of a rotor shaft, an irregular rotation of the rotor tends to occur to a great disadvantage. In order to compensate for such irregular rotation, an attempt is made to attach a balance weight on the rotor. In so doing, a clearance for the balance weight is provided between the rotor and a housing, thus making the housing bulkier.
An irregular rotation of the rotor often occurs due to, for example, a variation in input voltage to the motor. To avoid such disadvantage, a servo mechanism is provided by which the number of rotations of the rotor is controlled through its detection. The number of rotations of the rotor is normally detected by a permanent magnet mounted on the rotor and a detection coil mounted on that surface of the housing which oppositely faces the permanent magnet. This requires a spacing for such electrical parts or elements within the housing, resulting in a bulkier motor.